Internal combustion engines may utilize an exhaust system that includes a selective catalytic reduction (SCR) catalyst for reducing the amount of NOx that is ultimately discharged to the surrounding environment during operation of the engine. An SCR catalyst may utilize a liquid reductant such as an aqueous urea solution that is injected into the exhaust gasses upstream of the SCR catalyst. Prior to reaching the SCR catalyst, the water droplets within the injected solution may evaporate. The remaining urea component may then hydrolyze and decompose into ammonia which may then enter the SCR catalyst via the exhaust gas flow stream. A catalyst within the SCR catalyst may facilitate a reaction between the NOx component of the exhaust gas flow stream and the ammonia to break down the NOx into water vapor and nitrogen gas. The efficiency of this NOx reduction may be directly proportional to the degree of vaporization of the aqueous urea solution and uniformity of the distribution of the resulting ammonia (i.e. uniformity of ammonia concentration and uniformity of exhaust gas velocity over the cross-sectional flow area of the SCR catalyst) within the engine exhaust gasses upstream of the SCR catalyst.
Various systems have been used in attempts to address incomplete vaporization and unequal distribution of the liquid reductant within the exhaust gas flow stream prior to entering the SCR catalyst. For example, systems may utilize a mechanical device to increase vaporization and distribution of the liquid reductant within the exhaust gas flow stream. Typically, a system of this type will allow for an injected liquid reductant to impinge upon a mechanical device that may aid in the shearing of liquid reductant droplets. The mechanical device will typically be located within an exhaust flow stream, however, and may hence result in back pressure being imparted to the engine and resultant engine horsepower and/or torque losses. Additionally, such mechanical devices may form reductant deposits over time (e.g. melamine), which may eventually clog downstream exhaust passages and may impart an untenable amount of back pressure to the engine. Furthermore, although in-flow mechanical droplet shearing devices may result in improved reductant distribution, the dimensional constraints of such applications may not allow for sufficient mixing length between the injector of the liquid reductant and the SCR catalyst to achieve sufficient vaporization and uniform distribution of ammonia across the exhaust gas flow stream profile. For example, where the liquid reductant is not sufficiently vaporized by the exhaust gasses before reaching the catalyst within the SCR catalyst, drops of liquid may be deposited onto the catalyst, which may leave residue upon evaporation and eventually lead to degradation of the catalyst.
In one approach, a system for treating exhaust gasses from an engine, the exhaust gasses routed from the engine to atmosphere is provided. The system comprises an exhaust passage, the exhaust passage including a plurality of exhaust passages that separates the exhaust flow into smaller flows, wherein the smaller flows are introduced into a common passage and combine to form a single exhaust flow, and wherein the configuration of the plurality of exhaust passages impels the single exhaust flow to exhibit a helical flow pattern.
In this way, by inducing a helical flow pattern within an exhaust passage, angular momentum may be imparted to an exhaust stream flow that may increase reductant droplet shearing, vaporization, and uniform distribution of a liquid reductant within the exhaust gas stream flow. Additionally, the effective mixing length (and hence mixing time) for an exhaust flow stream and an injected liquid reductant may also be increased by utilizing such a configuration. Thus, the utilization of an exhaust passage that may induce a helical flow pattern within an exhaust gas flow stream may reduce backpressure on the engine, increase peak engine torque and horsepower, and increase SCR conversion efficiency via increased vaporization and uniformity of the distribution of the liquid reductant within the exhaust gas flow stream prior to entering the SCR catalyst while also reducing the formation of reductant droplets within the exhaust system.